I am from the Chicago area and have lived in the city proper for the last 13 years. Chicago is a very diverse city and has a variety of atmospheres. While the history isn't as long as your Spanish cities, it does show through. It is also modern and growing, especially in the city center. Lots of world class restaurants, museums, and entertainment options including a great music and theater scene.
Chicago is a city of neighborhoods which each have distinct histories and built environments. Chicago is known for having many tree-lined streets and parks so while it is densely populated, there is a connection to nature and the advantage of Lake Michigan which is basically a fresh-water sea. The Summer can get hot and humid, the winters can get very cold and snowy. There are some high-crime areas to avoid, but despite bad press lately, I generally feel safe, but you must be street-smart and aware of your surroundings like in any city.
I haven't ever been to Barcelona, but I was recently in Malaga and Madrid - Chicago falls somewhere between those two; Downtown Malaga reminded me a little of Chicago, but much smaller. Chicago and Madird are about the same area and population but Madrid has a different feel - much more densely built than Chicago for the most part, but Chicago has many more high-rises downtown. Also, unlike the Spanish cities I saw, our streets are on a gird system so it is easy to get around. Decent public transit in most areas too. Chicago isn't as bad as New York or LA, but it can be quite expensive; more expensive than Madrid these days for sure. Also unlike Spain, outside of the city there is sprawling suburbs in all directions. I don't care for most of them, but that's why I live in the city. Beyond the suburbs the region is very flat and mostly of corn fields and other agriculture.
To get a better idea I would suggest a virtual tour on google street view. Here are some places to start:
University of Chicago
Wicker Park neighborhood
Roscoe Village neighborhood